Alkylation of DNA in Drosophila melanogaster spermatozoa by 3H-ethyl-methanesulfonate (EMS) was determined as ethylations per nucleotide (dose) and compared to the relative frequency of sex-linked recessive lethal mutations induced with an equal exposure of non-labeled EMS. Alkylations accumulated on DNA in spermatozoa because of lack of repair mechanisms and persist until fertilization with the amount of alkylation being significantly reduced during early embryonic development. However, some alkylations remain on DNA throughout cleavage in the embryo. By feeding EMS for 24 hours in a concentration of 25mM, a high dose of 1.4 x 10 to the minus 2nd power alkylations per nucleotide was observed. With 1.4% of the nucleotides alkylated, 57% of the x-chromosomes were hemizygously viable; therefore, ethylation per se is not very efficient in inducing mutations. The relative frequency of mutations increased linearly with the dose from a dose of 2.1 x 10 to the minus 4th power 1.4 x 10 to the minus 2nd power alkylations per nucleotide. No threshold was apparent and the statistical limit of the exponent was 1.0 plus or minus 0.1. This linear relation suggests no change in mechanism of mutagenesis occurs from low to high dose in Drosophila. A nonlinear relation was found between exposure and dose; when exposure was increased by a factor of 250 (from 0.1 to 25mM EMS in the feeding medium), dose was increased by only a factor of 68. By extrapolating down from our lowest dose of 2.1 x 10 to the minus 4th power alkylations per nucleotide with an observed frequency of 0.55 plus or minus 0.08% sex linked recessive lethals, we estimate the doubling dose for sex-linked recessive lethals to be 4 x 10-5 alkalations per nucleotide. Current work is on establishing a dose response curve for methyl methanesulfonate (MMS).